Document that IntoIterator allows for `for...in` sugar

http://doc.rust-lang.org/core/iter/trait.IntoIterator.html

I think we should mention somewhere in the docs for `IntoIterator` that implementing this trait allows one to write:

``` rust
let my_vec = vec![1, 10, 100];
for i in my_vec { ... }
```

instead of

``` rust
let my_vec = vec![1, 10, 100];
for i in my_vec.iter() { ... }
```
